Generic ceramic pressure measurement cells have a ceramic measurement membrane and a ceramic counterpart, said measurement membrane is connected pressure-tight with the counterpart along a circumferential joint having an active brazing solder, wherein a pressure chamber is formed between the measurement membrane and the counterpart, wherein the equilibrium position of the measurement membrane arises from the difference between a pressure prevailing in the pressure chamber and a pressure acting on the outer side of the measurement membrane facing away from the pressure chamber. Generic pressure measurement cells also comprise a transducer for converting the pressure-dependent deformation of the measurement membrane into an electrical or optical signal. In the following, the pressure measurement cells with a capacitive transducer form the basis; this can equally be related to the invention of pressure measurement cells with that of other transducers.
In particular, aluminum oxide ceramics, which are suitable for the production of pressure measurement cells due to their elastic properties and their resistance to media, are used as the material for the base body and the measurement membrane. The ceramic components mentioned are joined, in particular, using an active brazing solder that preferably contains Zr—Ni—Ti. The production of such active brazing solder is disclosed, for example, in the European Published Patent Application EP 0490807 A2. According to the method described in the publication, rings that must be positioned between the measurement membrane and the base body to solder them together can especially be produced from the active brazing solder material.
However, the temperature range in which an active brazing solder forms a high-quality pressure-tight connection with a ceramic material is comparatively narrow.
At very low temperatures, the solder is not sufficiently reactive on the one hand and it is too viscous to spread evenly on a surface area to be wetted on the other. However, at too high temperatures, there is a risk that the solder has such a low viscosity that it enters areas that are not meant to be wetted by it.
However, in the production of larger batches of measurement cells, it is inevitable that a temperature distribution that exploits the available temperature range in an oven is given. Nevertheless, to obtain useful results, provision of a solder stop that limits the spread of the active solder is known.
To this end, for example, the published German patent application DE 100 36 433 A1 discloses a capacitive pressure measurement cell, which also has a joint with an active brazing solder, wherein on the base of the joint, namely the inner radius of the joint, an annular circumferential groove is formed, which prevents the localization of notch stresses at the joint on the one hand, and defines a reliable solder stop on the other, over which the active brazing solder cannot flow radially inwardly.
A practiced method for limiting the radially inwardly flowing active brazing solder is oxidation of the surface of a membrane-side electrode, which comprises tantalum, and should be in galvanic contact with the active brazing solder. At relatively low soldering temperatures, entry of the active brazing solder into the pressure chamber can be prevented with an acceptable yield. However, if the soldering temperature is increased, the solder stop no longer acts reliably, and the solder flows over the edge of the tantalum electrodes into the pressure chamber.